A vehicle's resistance to movement due to its rear section is known as base drag.
The fluid flows that have difficulty following the rear profiles of moving vehicles become turbulent behind the vehicle, which reduces pressure at the rear of the vehicle and creates a strong resistance to the vehicle's forward movement.
Passive solutions exist that reduce the rear drag of vehicles and in particular solutions are known wherein a vehicle's rear profile is realized as a cone by using an inflatable bladder as described for example in document DE 41 01 960, adding flow deflection profiles as described for example in document EP 0 273 850, one or more annular appendages as in document U.S. Pat. No. 6,297,486, lateral deflectors as in document U.S. Pat. No. 6,926,345.
Other realizations are based on active means such as mobile flaps as described in document U.S. Pat. No. 4,411,399 or an injection of fluid in the rear of the vehicle in order to fill the depression.
Aerial vehicles propelled by jet engines present a jet pipe propelling nozzle and the jet engines only create a small amount of drag because the jet of ejected gases plays a role in the vehicle's aerodynamic profile.
In contrast an unused engine creates an enormous amount of drag (up to one third of the vehicle's total drag).
That is why, for example, when the American space shuttle currently in service is transported by airplane, it is equipped with a conical rear cover masking the nozzles of its rocket engines.
On the other hand, such a cover cannot be used when this shuttle is launched because it would be necessary to jettison it before igniting the rocket engine, which would entail designing a cumbersome release device ensuring that no debris could damage the shuttle or the jettisonable boosters.
The same would apply to a space plane, for which the jettisoning operation would remain risky.
Similarly, using active fluid-injection devices on a space plane would require the fluid to be carried in the vehicle, which would reduce its payload.
Further, in the field of propulsion, it is known to fit turbine engines with active flow deflection devices to direct the flow of the propelling gases. Document US 2006/0150612 A1 is an example of such devices.
However, this technique is not used in rocket engines, for which directing the nozzle to adjust the vehicle's trajectory is preferred.
Similarly airbrake devices are known comprising flaps integrated in the aircraft jet engine's exhaust and being deployed to slow the aircraft down. Document U.S. Pat. No. 5,120,005 relates to such devices combining flaps opening behind the engine and flaps converging into the jet engine's exhaust flow to break the engine's thrust.
For its part, document FR 2 705 739 describes a device for adjusting the diameter of the exhaust section of a rocket engine divergent nozzle to adapt the divergent nozzle to variable ambient conditions of flight.
This device only changes the annular diameter of the exhaust and the shape of the walls facing the divergent nozzle along an axial symmetry in relation to the axis of the divergent nozzle.
As regards the stabilization of a towed vehicle by increasing its drag, it is known from document U.S. Pat. No. 5,871,173 to arrange blades that are deployed at the rear of the vehicle like umbrella stays.
The specific case of a spacecraft designed for atmospheric flight using jet engines and designed for flight outside the atmosphere using a rocket engine leads to contradictory requirements with respect to performance.
In aeronautical-type atmospheric flight, minimal drag is required and therefore in particular as small a cross-section as possible for the rear section, whereas for space flight, a large cross-section for the engine nozzle exhaust is desired, which is detrimental to the reduction of base drag.
The presence of rocket propulsion therefore presents a problem in atmospheric flight because it generates a lot of drag due to the cross-section of the nozzle which is wide and forms a straight cut at the rear of the vehicle.